The basic operational concepts of the internal combustion engine have remained largely unchanged for much of the past 130 years, since the patent issued to Karl Benz in 1886. Any improvement in the efficiency of internal combustion engines is highly desirable, on the grounds of direct and indirect costs to the user and to the environment.
Engine efficiency, defined as the work done per unit of fuel used, may be improved by addressing input or output aspects of the combustion process. One well known approach to address input aspects of the combustion process to improve efficiency in gasoline-fueled engines is the development of direct injection gasoline engines, in which the two functions of fuel introduction and air introduction into the engine cylinders are separated.
However, even when direct fuel injection is used with conventional four-stroke engines, whether gasoline or diesel-fueled, factors remain that significantly limit engine efficiency. One such factor is incomplete flushing of residual exhaust gas by the exhaust stroke, which results in undesirable dilution of the fuel/air mixture after the following intake and compression strokes. This in turn not only lowers the concentration of the newly introduced combustable gas mixture but also reduces the temperature of the mixture, both effects resulting in reducing the total energy generated from subsequent combustion. A second factor, which applies to direct injection gasoline engines but not to diesel engines, is incomplete mixing of the fuel/air mixture in the cylinder before the compression stroke, the resulting spatial non-uniformity of the mixture creating obvious problems in achieving repeatable, predictable combustion. Variations of +/−10% in the combustion energy conversion from chemical to thermal energy are typical, forcing sub-optimal choices of both ignition timing and the amount of fuel required to be injected. In practice, to ensure that the fuel-poor regions within the cylinder will still experience combustion, significantly more fuel is introduced than would be necessary if it could be assumed that the fuel would be uniformly distributed throughout the cylinder space before ignition.
There is therefore a need for methods and systems to improve the efficiency of internal combustion engines by addressing the problems of incomplete flushing (common to both direct fuel-injected gasoline engines and diesel engines) and of incomplete mixing (present in gasoline engines). Such methods and systems would ideally require relatively small changes to engine design and operation, the direct and indirect costs of those changes being outweighed by an accompanying increase in the efficiency of fuel usage.